Many households presently have several telephone extensions distributed throughout the house connected to a single telephone line. The extension telephones enable a party to answer or initiate a call from any of the telephone locations. The convenience of an extension telephone is enhanced by the use of an intercom system which enables the answering party to call another person to the phone by "paging" that person over the intercom rather than by shouting aloud for that person. Some home telephone adjuncts or intercom systems also provide a capability to allow the answering party to return the handset to the on-hook position and then pick up the call at another telephone location without dropping the call. Such a "hold" feature together with a "paging" capability are part of the prior intercom art. Additionally, some prior intercom art allows for signaling a busy intercom connection that an outside call is ringing (call waiting feature).
These hold, paging, and call waiting features also are available in the commercial environment in the form of key telephone systems. These key systems, however, are not always practical for single-line installations since they require key telephones, line circuits and additional wiring to provide the required functions. An intercom system to be used with a single-line installation must be economically justifiable, and thus ideally should use the existing nonkey telephones and four-conductor telephone cable that is prewired in homes.
The problem with the prior art intercom systems which are used in conjunction with an existing single-line telephone system is that some intercoms require more than the standard four-conductor telephone cable to provide an intercom function. Additionally, some intercoms require modifications to the existing telephones, while other intercoms require the addition of pushbuttons or switches to activate and release the hold, paging or call waiting features. These prior art intercom systems require separate operations to activate and release the various system features or require the use of the telephone switchhook for signaling, which is confusing and could result in dropping an active call. Thus a need exists to provide residential users with an economic intercom system which provides hold, paging and call waiting features yet is both simple to operate and reliable in its operation.
Accordingly, it is an object of my invention to simplify intercom operation by consolidating the multiple operations required to enable and disable the system features into a single pushbutton operation without using undesirable switchhook signaling.
It is another object of my invention to use existing telephones and 4-wire telephone cable as part of the intercom to minimize system cost.
It is another object of my invention to revert to normal telephone service during a commercial ac power failure at the intercom location.
It is a further object of my invention to provide positive visual feedback of system status information at each adjunct without the use of line cards or additional wiring.